The invention relates to a photoalignment composition comprising at least a single photoalignment polymer, preferably a homo- or a copolymer having at least one photoreactive group; and at least a single solvent of formula (I)(R—CO—O—R′),
wherein R and R′ are independently from each other straight-chain or branched alkyl chain, with the proviso that at least one alkyl chain is branched.
Further, the present invention relates to the use of this composition for coating or printing, especially for printing, more especially for inkjet or offset printing processes, and most especially for an offset printing process. In addition, the present invention relates to the use of the photoalignment composition for the preparation of liquid crystal aligning layers and in the construction of unstructured and structured optical elements and multi-layer systems, especially liquid crystal displays, LCDs.
In general photoalignment materials are applied by two main techniques, the coating and printing.
Coating methods are for example spin coating, air doctor coating, blade coating, knife coating, reverse-roll coating, transfer roll coating, gravure roll coating, kiss roll coating, cast coating, spray coating, slot-orifice coating, calendar coating, electrodepositing coating, dip coating or die coating.
Printing methods are for example relief printing such as flexographic printing, intaglio printing such as direct gravure printing or offset gravure printing, lithographic printing such as offset printing, or stencil printing such as screen printing.
In general, there are five major printing processes which are distinguished by the method of image transfer and by the general type of image carrier employed.
Depending upon the process, the printed image is transferred to the substrate either directly or indirectly. In direct printing the image is transferred directly from the image carrier to the substrate, examples of direct printing are gravure, flexography, screen printing and letterpress printing processes. In indirect, or offset, printing, the image is first transferred from the image carrier to the blanket cylinder and then to the substrate. Lithography, currently the dominant printing technology, is an indirect (offset) process.
Image carriers (or plates) can generally be classified as one of four types: relief, planographic, intaglio, or screen. In relief printing, the image or printing area is raised above the no image areas. Of the five major printing processes, those relying on relief printing are letterpress and flexography. In planographic printing, the image and no image areas are on the same plane. The image and no image areas are defined by differing physiochemical properties. Lithography is a planographic process. In the intaglio process, the nonprinting area is at a common surface level with the substrate while the printing area, consisting of minute etched or engraved wells of differing depth and/or size, is recessed. Gravure is an intaglio process. In the screen process (also known as porous printing), the image is transferred to the substrate by pushing ink through a porous mesh which carries the pictorial or typographic image.
Inkjet printing represents a further established and strongly growing industrial image deposition process which is also called printing despite its quite different character (e.g. its lack of an image carrier) in comparison to the above described conventional printing processes. Inkjet printers may operate either in continuous or drop-on-demand (DOD) mode. In continuous-mode, the fluid is pumped through a nozzle to form a liquid jet. Uniformly spaced and sized droplets are obtained by imposing a periodic perturbation, leading to surface-tension driven jet break-up. This requires the fluid to have certain conductivity. Drop-on-demand is the method of choice for many industrial applications such as electronics and displays. This is mainly because of its smaller drop size, higher accuracy and no need to use dopants to make the fluid more conductive. An acoustic pulse ejects fluid droplets from a reservoir through a nozzle. The pulse can be generated either thermally (Bubble-jet) or piezo-electrically (piezo-jet). Piezo-electric DOD is generally more suited to a variety of solvents. The most critical part of inkjet printing technology is probably the fluid and its physical properties.
The fluid parameters should be on one hand optimised to get reliable droplet formation during printing with specific printer (such as viscosity and surface tension). The fluid should be also optimised in order to get good wetting properties, with specific substrates, and homogeneous films.
Flexographic technology is successfully used to apply liquid crystal aligning layers in the LCD industry. According to the flexographic technology, the liquid crystal aligning layers can be directly printed in a predetermined pattern without requiring any etching process. Generally described, the flexographic process can be a substantially continuous process which uses a series of rollers to transfer a quantity of alignment material as a predetermined pattern onto the desired surface of substrate (typically glass). Stated differently, the flexographic apparatus can be configured to automatically convey a plurality of glass panels sequentially through the process. The apparatus employs a special resin plate with a series of holding cells configured in a predetermined pattern to hold a quantity of alignment material processing solution. The resin plate is rotated and subsequently brought into direct contact with the glass panel surface to release the solution and transfer the predetermined pattern to form the liquid crystal aligning layers onto the surface. The glass panel is then heated to pre-cure the liquid crystal aligning layers.
In more detail the fabrication of a liquid crystal aligning layer by means of flexographic printing is described in the following. The alignment material solution is dropped and maintained on a so-called anilox roll. The anilox roll is preferably plated with chrome, or chrome and nickel (the alignment material solution contacting surface). Cells having a pyramid shape are formed in the anilox roll to a depth of approximately 20 μm so that the alignment material can be retained in the cells.
A doctor roll is rotated together with the anilox roll to help assure that the alignment material solution is evenly applied on the surface of the anilox roll. A resin plate is attached to an underlying plate typically comprising copper. In a preferred embodiment, the resin plate is formed from a polybutadiene resin, e.g., an Asahi Kasei photosensitive resin (“APR”). For ease of description, the resin plate will be described subsequently herein as an “APR plate”.
The APR plate includes an exterior surface of predetermined patterns. The patterns include a plurality of cells as intaglios so that the alignment material solution can be accommodated in the cells. The depth of the cells is preferably in the range of approximately 15 to 20 μm. Thus, in contrast to the above general description the flexographic printers for liquid crystal aligning layer application use an intaglio instead of a relief process.
The alignment material solution is dispensed onto the anilox roll. The anilox roll and the doctor roll rotate and contact on an outer circumference portion. The contact pressure provided by the doctor roll helps the alignment material solution in the anilox roll (i.e., in the cells) to be uniformly distributed and to bring a certain quantity of the solution to fill the cell of the anilox roll. The APR plate then contacts the anilox roll and the alignment material solution is supplied or delivered to the APR plate. Typically, the amount of the alignment material solution transferred to the APR plate is determined by the contact force that the doctor roll exerts against the anilox roll (or the reverse). The alignment material solution is then held in the APR plate cells.
The LCD panel or glass is fixed to the print table. The table advances and the glass is moved forward at the same time that the copper plate holding the APR plate rotates. The APR plate thus directly contacts the glass substrate and the alignment material solution is applied to the glass from the cells of the patterns in the APR plate. As a result, an orientation layer is formed on the glass substrate.
There is an enormous demand of photoalignment compositions since these materials give access to very efficient and economic large scale manufacturing processes. However, their applicability, such as their printabilty is often not satisfying with the high industrial standards. Here are high quality standards required, such as even wetting properties; a desired thickness of the applied layer, homogeneity over the whole surface of the layer is required.
Therefore, is a constant need for photoalignment materials allowing good coatability or printability for the fabrication of thin liquid crystal aligning layers which fulfil the requirement of uniform defect free surfaces.